The present invention relates generally to magnetic resonance (MR) imaging and, more particularly, to directly measuring cross-axis eddy currents for calibrating MR scanners. As cross-axis currents can contribute to less than ideal image quality, the present invention overcomes the drawbacks typically associated with conventional on-axis only and cross-axis measurement and scanner calibration techniques.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
In several MR data acquisition techniques, positive and negative gradient pulses are applied to spatially encode the signal emitted by excited spins. This switching of the gradient pulses between positive polarity and negative polarity has been found to produce eddy currents. Conventionally, it has been assumed that the eddy currents manifested themselves on the same axis on which the gradients were being switched. For example, if gradient switching is carried out on the Gx axis, it has heretofore been assumed that any eddy currents produced by the switching on the Gx axis will also manifest themselves on that same Gx axis. However, as gradient slew-rates have increased, it has been found that eddy currents manifest themselves on the cross axes, e.g., Gy or Gz.
Specifically, pulsed gradient waveforms applied on one axis have been found to produce eddy currents on the orthogonal axes. These cross-axis eddy currents are a superposition of eddy currents with a wide range of time constants and have been found to slow the response time of gradient coils thereby negatively affect gradient coil performance. A number of techniques have been proposed to reduce the impact of on-axis and cross-axis eddy currents; however, these proposed techniques are believed to be unreliable for measuring cross-axis eddy currents having short time constants.
It would therefore be desirable to have a system and method capable of directly measuring cross-axis eddy currents that is also reliable for such eddy currents having short time constants.